program Project2;

{$APPTYPE CONSOLE}

uses
  SysUtils;

var
  n, i, max: integer;
begin
  readln(n);
  max:=0;
  if (n mod 2 = 0) then
    for i:=1 to n do
      if (i mod 2 = 0) then
        max:=max+i;
  if (n mod 2 = 1) then
  begin
    max:=3;
    for i:= 5 to n do
      if (i mod 2 = 1) then
        max:=max+i;
  end;
  writeln(max);
  readln;
end.
 